High-voltage pulse-generator circuits are useful in a wide variety of applications, from long-distance radio communications to intricate surgical procedures involving the application of high-intensity pulsed electric fields to the lens of the human eye. Many of these applications require tight control of the pulse shapes and durations, even in the face of wide variations in the characteristics of the loads receiving the pulses.
Simple high-voltage pulse generators using transmission-line devices for energy storage have been used for several decades. One such device, pictured in FIG. 1, was described by Ishii, et al., in the article “Self-matched high-voltage rectangular wave pulse generator,” published November 1985 in the Review of Scientific Instruments, vol. 81(11). A similar device, using a spark discharge device in place of the switch S1 pictured in FIG. 1, is described in U.S. Pat. No. 4,536,723, issued 20 Aug. 1985 to Lang et al., and titled “High-Power Pulse Generator Using Transmission Line with Spark Discharge Device.” The entire contents of U.S. Pat. No. 4,536,723 are incorporated by reference herein, to provide background for the below description of improved pulse-generator circuits.
The detailed operation of the circuit of FIG. 1 is described in the Ishii article. Transmission line 100 serves as a capacitive energy-storage device. Here, transmission line 100 is illustrated as a segment of coaxial cable, although other transmission lines might also be used. The coaxial cable segment has an inner conductor and an outer conductor, with suitable insulation between the conductors. In the illustrated circuit, the outer conductor of transmission line 100 is charged to voltage VSUPPLY by power supply 110, through charging resistor RC. A terminating resistor RT is connected to the inner conductor at one end of transmission line 100; for best operation (i.e., to minimize pulse reflections propagating towards the load), the value of RT is selected to match the characteristic impedance Z0 of transmission line 100. Thus, for example, a 50 ohm resistor should be used to terminate a coaxial cable segment having a nominal characteristic impedance of 50 ohms. The other end of the inner conductor of transmission line 100 is connected to the load ZL—as discussed in further detail below, the operation of this pulse-generator circuit is generally insensitive to the impedance of load ZL, insofar as reflection-free operation is concerned.
When the outer conductor of transmission line 100 is charged to VSUPPLY, the closing of switch S1 simultaneously shorts both ends of the outer conductor to ground, initiating the simultaneous launch of traveling waves from both ends of the transmission line towards its center. If VSUPPLY=−2 (an assumption that simplifies the following expressions), the traveling wave launched from the load end of transmission line 100 has an amplitude of (α1−2), where the refraction coefficient α1 equals 2ZL/(Z0+ZL), and Z0 is the characteristic impedance of transmission line 100. The traveling wave launched from the other end has an amplitude of (α2−2), where α2=2RT(Z0+RT). If the terminator resistor RT is selected to match the characteristic impedance of transmission line 100, then α2=1.
With the simplifying assumption that transmission line 100 is lossless (and assuming that RT is matched to Z0), it can be shown that the voltage across the load ZL, relative to the switch's closing at time t=0, is given by:
  {                                                                        V                L                            =                              α                1                                      ,                                                              for              ⁢                                                          ⁢              t                        =                          0              ⁢                                                          ⁢              to              ⁢                                                          ⁢              τ                                                                                                      V                L                            =              0                        ,                                                otherwise            ,                                   
where τ is the electrical length of transmission line 100. In other words, the voltage waveform across ZL is a simple rectangular pulse having an amplitude of α1 and a duration of τ. Importantly, the pulse's duration, which is established solely by the electrical length of the transmission line, is independent of the impedance of the load ZL.
Those skilled in the art will understand that the inner and outer conductors of transmission line 100 are electrically interchangeable. Thus, the components of FIG. 1 that are connected to the outer conductor of transmission line 100 may be instead connected to the inner conductor of transmission line 100, and vice-versa, without changing the basic operation of the circuit. Of course, the practical impact may be much more significant; thus the pictured configuration may be more convenient to implement. The same is true with respect to all of the schematic illustrations provided herein—while those illustrations suggest particular configurations with respect to the inner and outer conductors of one or more coaxial transmission lines, those skilled in the art will appreciate that the conductors of a transmission line are generally electrically, if not physically, interchangeable.